Drop thumb type from (most) movies, shows, seasons + sets + fixes

[jmarshallnz]

The main thing that this PR handles is dropping the "thumb" art type from movies, shows, seasons and sets. Essentially we demand a poster for movies and sets, and a banner or poster for shows and seasons.

It works as follows:

1. Local art, if known (poster.jpg etc.) we assign directly.
2. Local art, if unknown (thumb.jpg, *.tbn etc.) we cache, and based on the aspect ratio determine poster or banner status and assign accordingly. If in-between, we cache using thumb normally.
3. Online art, we assign directly if the aspect attribute is specified.
4. Online art, we assign to the preferred type (poster for movie, banner for shows) if no aspect attribute is specified.

There's update code in place to move all the existing thumbs to poster/banner (both Eden + pre-Frodo).

Finally, thumb is always available as a fallback for skinners (it falls "forward" to poster then banner if a thumb type isn't there).

In addition, this moves to support banner and poster (and fanart) for seasons.

Finally, we have a bunch of fixes for the "Season Thumb" and "Movie set thumb/fanart" context menu items, which have been replaced with a "Choose art" item, which throws up the Choose Art dialog.